Given the dramatic changes in the earth's atmosphere, precipitated by industrialization and natural sources, as well as the dramatically increasing number of household and urban pollution sources, the need for accurate and continuous air quality monitoring has become necessary to both identify the sources and warn consumers of impending danger. Tantamount to making real-time monitoring and exposure assessment a reality is the ability to deliver, low cost, small form factor, and low power devices which can be integrated into the broadest range of platforms and applications.
There are multiple methods of sensing distinct low density materials such as gasses. Common methods include gas chromatography, nondispersive infrared spectroscopy (NDIR), the use of metal oxide sensors, the use chemiresistors, and the use of electrochemical sensors. The present invention pertains to electrochemical sensors. The principle of operation of an electrochemical sensor is well known and is summarized in the following overview: http://www.spec-sensors.com/wp-content/uploads/2016/05/SPEC-Sensor-Operation-Overview.pdf, incorporated herein by reference.
Basically, in an electrochemical sensor, a porous sensor electrode (also known as a working electrode) contacts a suitable electrolyte. The gas permeates the electrode and contacts the electrolyte. The sensor electrode typically comprises a catalytic metal that reacts with the target gas and electrolyte to release or accept electrons, which creates a characteristic current in the electrolyte when the electrode is properly biased and when used in conjunction with an appropriate counter-electrode. The current is generally proportional to the amount of target gas contacting the sensor electrode. By using a sensor electrode material and bias that is appropriate for the particular gas to be detected, the concentration of the target gas in the ambient atmosphere can be determined from the sensing current.
The sensitivity of an electrochemical cell to a particular gas may be impacted by the application of a bias voltage to that cell. Therefore, by applying a set of different biases to an electrochemical cell, and comparing the recorded signal to a library of signals (in a look-up table) corresponding to those signals characteristic of individual known gasses, it is possible to ascertain the presence of, differentiate between, and quantify the occurrence of multiple gasses in the environment of the sensor. By continuously and quickly ramping the bias applied to the cell, a single electrochemical cell may rapidly differentiate between multiple gasses in its environment. Identification of an analyte in a controlled environment, such as that of a laboratory, via a potential ramping scheme is known in the art as “voltammetry”. However, electrochemical sensing systems known in the art are limited with respect to the speed at which voltammetry may run due to a combination of the electrochemical cells having large resistance-capaciotance (RC) time constants and the drive electronics having long settling times, resulting in voltammetry measurements taking up to 20 minutes or even more. Ramping the bias voltage at only a few millivolts per second is typical.
In the event that such a measurement is performed in a highly controlled environment, such a long data collection time is of no consequence since the test conditions may be controlled over that timeframe. However, outside of a controlled laboratory environment, the ambient conditions of the cell may change significantly over the course of such an extended period of data collection. For example, in an everyday consumer use case, the ambient relative humidity, the ambient temperature, and the ambient mixture and concentrations of analyte gasses may change significantly over such an extended data gathering period. All these factors combined with electronic drift inherent to running voltammetry on the electrochemical cell render analysis of the data meaningless. Further, if the gas sensor is intended detect dangerous gasses, the long delay time may result in a harmful effect.
Accordingly, for such a consumer application, an electrochemical gas sensor is required having small form factor, and hence a small time constant, such as described in the Applicant's U.S. patent application Ser. No. 15/598,228, in conjunction with a novel drive circuit enabling short settling times. Such a system enables voltammetry to be performed in a period of a few seconds or less. Over such a timescale, the ambient conditions in a majority of uncontrolled consumer environments in which the cell may be present would be essentially invariant, resulting in the ability to perform voltammetry in an essentially uncontrolled environment whilst providing data of sufficient quality to enable accurate electrochemical analysis.
The varying bias voltage may be sinusoidal or have another waveform. It is common to supply the variable bias voltage via a digital-to-analog converter (DAC) that outputs a varying voltage. Such an output contains discrete steps due to the quantized nature of the DAC output, and thus there is a high dv/dt. The rate at which the bias may be ramped in an electrochemical cell is limited, among other factors, by the capacitance of the cell, and the presence of current spikes occurring within the cell during the step-wise application of the bias ramp to the cell. The impact of the capacitive nature of the cell on the current spikes occurring during a voltage step is described by the equation i=C dv/dt. Excessive transient currents can lead to damage of the cell. In certain schemes, accurate measurement of the cell requires for the transient currents to have mostly decayed so that the cell output is measured while in its steady-state condition. Accordingly, a settling time typically occurs between the point at which a voltage step is applied to the cell and the point at which the current generated at the working electrode of the cell is measured. This limits the rate at which a bias sweep of the cell may be applied. Minimizing the capacitance of the cell and occurrence of current spikes within it allows for maximizing the rate at which voltammetry may be performed, hence maximizing performance of the sensor.
Electronic noise is also an important issue in electrochemical sensors which affects accuracy and speed achievable by the sensors. Electrochemical sensors are known for picking up 60 Hz and a variety of RF noise due to the large electrodes and electrolytes. Commercial systems can minimize the noise through shielded housings around the sensors, powering the sensor using a battery, and using signal processing to filter out the noise. However, noisy environments still generally limit the accuracy of the electrochemical systems and size of the systems due to the extra electronics needed to minimize the noise. This impacts the rate at which the ramps of the voltammetry can be executed at due to higher current spikes and longer settling times which can occur during step transitions due to the additional circuit elements, such as resistors and capacitors, which need to be added to mitigate noise.
Additionally, any flexure of the working electrode changes its electrical characteristics. Flexure may occur due to gas pressure fluctuations. Pressure built up due to small pore size in the electrode can also change the position/formation of the three phase interface inside the electrode which can impact the sensitivity of the sensor. The three phase interface is a critical aspect of a gas diffusion electrode and is formed at the conjunction interface of a gas, solid, and liquid (i.e., gas/electrode/electrolyte). A change in pressure could influence the density of the three-phase interface and where the three-phase interface is formed.
Accordingly, what is needed is an electrochemical sensor system for gasses that employs a varying bias signal that does not have high dv/dt characteristics. What is also needed is an electrochemical sensor that is less susceptible to noise. Additionally, an electrochemical sensor where the working electrode does not substantially flex due to changing gas pressures is required for stability in changing environments. Further, a control method is desired for use with a low capacitance gas sensor that can quickly detect a variety of different gasses in a short time.